Please note that one of the biggest annoyances I have when dealing with much of this type of information is trying to make it too technical for the audience. I know that most of the readers of this blog are not electrical engineers, nor do they aspire to be. They are musicians who are looking for answers or just killing time. Because of this, I'm keeping this simple and high-level. I'm going to avoid the very involved math that makes this work. I'm going to sacrifice pure technical accuracy for general understanding. The descriptions and examples do not apply to 100% of the situations and circumstances out there, so if this isn't exactly your brand of mojo, it's ok, I understand.
Basics
There is no black magic involved in modern musical instrument design and amplification. The concepts employed in making our Strats and Les Pauls loud are, at their heart, coming up on 100 years old. This means that these were some of the absolute first things discovered in modern electronics. I'm gonna burst your bubble at the right now, but if you think there's something special about this whole thing, you're mistaken. I'm gonna tell you why…
The Guitar
The guitar is the key part of the chain. It represents the user input and is a highly flexible input device at that. It is essentially the analog of the touch screen on your smartphone or the keyboard and mouse on your computer. Kinda sucks the fun out of music doesn't it?
On an acoustic guitar, we have an arrangement of strings. Each of these strings are tuned to a different tension, which means they will vibrate at a different speed resulting in a change in pitch. As we shorten or lengthen the string, the speed of vibration changes and the pitch changes accordingly. When the string is halved in length, it vibrates exactly twice as fast and the resulting pitch is twice as high. That's basic physics of a vibrating string and everything stems from this principle. The acoustic guitar is designed to make things loud based on the design of the body and acoustic resonance. Different wood types, body shapes, and construction all go into making the instrument more resonant providing volume and sustain, which is the property of allowing the string to vibrate longer.
An electric guitar (or bass) has most of these same concepts. The designer still needs to allow the strings to vibrate and sustain is a concern as well. Different designers have created different shapes and construction techniques to help everything move along. However, the one big difference is that an electric guitar does not rely on the guitar itself for amplification. Amplification is provided through a dedicated device such as an amp or PA or other mechanism.
So, the challenge comes in the question of how do we turn physical motion into electrical impulse to provide input into something to make it louder? This is where things get pretty cool. Years ago, some very observant people realized that if you take a magnet and wrap it in wire, you can create an electrical impulse which is known as induction. However, if you take that inductor and modify the magnetic field, you will modify the electrical impulse generated. Breaking this down, if we have a magnet or group of magnets and then wrap them with copper wire, we have a very basic pickup. Using steel strings, we can vibrate those strings by playing them on the guitar, and create a changing electrical current. Voila, we now have converted physical motion (vibrating guitar strings) into electrical impulses.
But Brad, why does my ______ wood guitar sound better than my ______ wood guitar?
This is where that all comes back to the instrument designer. Is wood a conductor of electricity? No, of course it isn't. Simply by long held laws of electricity, we can logically conclude that your Mahogany, Ash, Maple, Pine, or whatever body has absolutely no direct bearing over your amplified sound. Before the flames start, I said DIRECT bearing. Wood does not get picked up by a magnetic inductor no matter how hard you try. However, indirectly, the wood will lend itself to the resonance of the guitar itself and help the strings vibrate longer and decay in different ways. This is how wood affects tone. Unfortunately, not all guitars are created equal, therefore there's more than just wood involved, but that's the short answer.
Guitar Controls
Once we have a good transfer of energy from one form to another, we could very happily send it on its way to the amplification device. There is nothing that stops us from wiring the negative and positive of the pickup to the jack and calling it a day. But, there are things we really have grown accustomed to over the years, so we need to add them.
A basic inductor only needs two poles to function, positive and negative. If we look at an old vintage single coil from the 50's, you will notice only two wires. This does work, however we have grown used to certain things. Ground wires. We like ground wires. We probably remember from elementary school that electric likes to find its way to the ground as is the case with lightening and power lines. In our case, not all electric impulses generated are good impulses. Sometimes we get high frequency impulses that are introduced into the system, like the dreaded 60 cycle hum as well as others. A good ground system will allow many of these undesirable frequencies to find their way out before reaching the amplifier and also getting louder. The major drawback to our system is that we have a very self-contained electrical generation system surrounded by non-conductive wood held by a person above the ground. There is no direct path … except there is - you. We usually tie the ground back to the bridge or other metal piece that channels the path to the strings. When you play the guitar and make signal, the ground path is complete through you. Luckily, we're dealing with very small voltages, often less than 1 volt and usually closer to .5 or .75 volts at the very upper end of the spectrum.
With a pickup and a ground we have some level of silence, but this is pretty useful. Something even the most ardent minimalists use is a volume control. This is wired in between the pickup and the output jack and takes the form of 250k, 500k, or even a 1m potentiometer. A pot is nothing more than a variable resistive component that, as it moves, increases or decreases the resistance to the signal from 0% all the way to 100%. That's a mouthful, eh? Think of it like a faucet - you turn the handle in one direction and the water is off while in the other direction it's fully open. A volume pot works the same way just with a small difference. Water can happily be held back unless there is steadily increasing pressure behind it. Electricity needs somewhere to go. In this case, the volume pot slowly redirects signal to ground. When the volume is all the way "up," the signal is 100% (ish) going to the output jack. When the pot is all the way "down," the signal is going all to ground. Electricity follows the path of least resistance.
The above graphic is my miserable attempt at a free-hand drawing. The three components shown are the pickup, the volume pot, and the output jack. The red line is the hot from the pickup to the first lug of the volume pot. The blue line is the output wire from the center lug or wiper of the pot to the tip if the output jack. The green lines are grounds. Notice that the pickup is grounded, the volume pot's third lug is grounded, and the sleeve of the output jack is grounded. This is about as simple as things get.
Brad, I bought these super gee-whiz branded pots from expensivemusicsupply.com and they are the best!
This is one of my pet peeves. I don't care where you bought your pot or how much you paid, a pot is a pot is a pot is a pot. There are very few things that make a difference in electronic components. One of these is construction quality, and unless you are buying mil spec parts that are manufactured under very tight tolerances, your pot probably came from china like all the rest. But, is build quality the end all be all? There's also component error. If you buy a 500k tone pot, do you know the error on it? Components are not manufactured to exact specs therefore they are built with tolerances in mind. A pot may be a 10% tolerance. That means it's acceptable for the 500k pot to be anywhere from 450k all the way to 550k (500k x 10% = 50k +/-). However, it's just as common to find 20% tolerance in this type of component, which means your 500k could be anywhere from 400k to 600k. This also means that a simple component swap with the same printed value could yield anywhere from 100k to 200k difference. This is just the variance on a single pot. If you add in a Tone pot to the mix, you could have a 40% variance on the two values. In other words, although you might expect a 1000k total resistance across the two pots, you may find it's anywhere from 800k all the way to 1200k. If you are playing a Gibson Les Paul style guitar with two volumes and two tones, your expected 2000k load could be anywhere from 1600k to 2400k. These are significant differences that will have a noticeable effect. My best advice is to look at your old 500k pot with a meter and then look at your new 500k pot with a meter and see the difference. You will probably be surprised. Many times when you hear other musicians rave about how a simple change of pots brought their guitar to life, it's just dumb luck.
Are high end "specialty" products worth while? Nope - in most cases you can find better pricing with regular components. Check out a place like Digikey that's not trying to bilk musicians out of money for snake oil and you will see what component prices are really like.
Tone Controls
Let me wrap up today's post with a little discussion on tone controls. There are very few things that bring out the crazy like tone controls and what they do.
Very simply, a tone control is another variable resistor (pot) that will selectively add resistance to frequencies within the signal to change what gets sent to the output jack. This means that through the use of a capacitor, the tone control can isolate a certain frequency threshold and send it to ground. This is one of the cool features of capacitors that are used extensively in audio reproduction and amplification.
As we remember from the above discussion on volume pots, when we turn down the pot, we are actually adding resistance to the signal, which sends more and more of the electrical impulse to ground. This effectively shuts off the faucet. A tone control works the same way except it doesn't affect the entire signal, it usually only works on the high frequency portion of the signal. The capacitor is wired inline with the volume and tone and acts as a gate keeper. The lower the value of the capacitor, the higher the frequency that gets affected. A common value is either a .022 or a .047 mf or micro farad capacitor. These values look at the whole signal and essentially say that anything past a certain frequency will get sent to ground. This gives the effect of accentuating the bass tones more as the tone control is rolled off.
Here is drawing number two showing the simple innards of a guitar. Notice the same basic scheme as the above simple volume only diagram. Here, I've simply added a tone pot. We can see that the red lead from the pickup goes to both the volume and the wiper (middle) lug of the tone pot. This allows the circuit to utilize the resistance of both pots in tandem. Additionally, notice that there is a capacitor on the other lug of the tone pot, which grounds it to the pot enclosure and then the pot itself goes to ground. This allows the signal to "bleed" certain frequencies of the output to ground and reduce high frequencies as the pot is turned down.
So, back to an earlier discussion, does the capacitor brand make a difference? Does it make sense to purchase a $10 or more specialty cap to improve your tone? I will say yes and no. Yes, only if it is a cosmetic change that will superficially help you obtain the desired aesthetics. No in almost any other situation.
A capacitor is a device that, much like a resistor or pot, is manufactured to certain specifications. It has a very well defined purpose in modern electronics. One of which is to attenuate frequency as well as level out current inconsistencies. Various capacitors have various designs and construction, but almost all of that is based on power handling and tolerance. Whether you have a paper cap, ceramic cap, oil cap, or any of the others that are produced, they all do the same thing in the same way at the same value. There is no tonal difference in capacitors of the same value.
However, once again, we get to tolerance. Some caps have a 1% tolerance. That means that a .022 mf cap will fluctuate between .02222 and .02178. These are pretty inconsequential values. The problem is using a cap with 5% or 10% tolerance, as is the case in some older designs. A 10% tolerance gives a high end of .0242 and .0198. These still aren't too bad, but then the .022 cap is not a huge tone difference. On the other hand, if we look at the other common value of a .047, a variation of 10% is .0047 giving a high value of .0517 and a low of .0423. This is a pretty significant swing. This means that what could have been a perfect balance to your ear is just a little too much. Although not always, many of the "vintage-like" products are designed and sold in their original state, which may have very high variances. Not to mention, some of the antique components are actually lesser quality and add undesirable effects such as noise and hum and have a very finite lifespan.
Again, before just blindly falling for snake oil, get out your meter and measure these things. Don't just change parts blindly, but do it with some sense of purpose. The starting point it to know what you have now and change accordingly. Just because you paid decent money for a cap at stewmac or all parts or wd or wherever, doesn't mean the part isn't crap. Electronics is not black magic and it's a very well defined science with very reliable parts and components that are very cheap from real suppliers. Don't buy the hype - get good information and know why you are doing things.
Hopefully this gave a little overview of the first part of the chain of tone and why things work the way they do. In the next installment, I will cover the guitar output, the cable, the input jack, and the input stage of an amplifier. Should be fun and exciting.
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